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Hair is the filamentous outgrowth of the epidermis found in mammals. Hair is a characteristic of all mammals, though in some species hair is absent at certain stages of life. "Hairs" are also found on plants, the technical term for which is trichomes (see for further discussion of plant hairs).
Hair serves a number of different functions. It provides insulation from cold weather and, in some species, from particularly hot weather. Because hair is often pigmented, it provides coloration. This might serve to camouflage an individual; in some mammals, the pigmentation changes with the seasons, becoming white during the snowy winter, for example.
In modern Western societies, it is considered manly for men to have hair on their faces, arms, chests and legs, but the hair growing from the top of the head is generally kept short, relatively speaking; equally, it is considered womanly for women to have no hair on their bodies, with the exception of pubic hair, but to have a lot of it on the tops of their heads. This is a fairly recent development. Before the First World War men generally had long hair and beards. The trench warfare between 1914 and 1918 exposed men to lice and flea infestation which caused the order to be given for the routine cutting of hair to a severely short length. The shorter style became the new normality and has never entirely gone away since.
The hair of non-human animal species is commonly referred to as fur.
One theory suggests that nature selected humans for little body hair as part of a set of adaptations including bipedal locomotion and an upright posture. Bipedal locomotion is extremely inefficient, and many animals can outrun humanHuman beings are defined variously in biological, spiritual, and cultural terms, or in combinations thereof. Biologically, they are classified as Homo sapiens ( Latin for knowing man , a primate species of mammal with a highly developed brain. In spiritua beings for short periods of time; such animals, however, are inefficient radiators of heat, and cannot run for long periods of time (notable exceptions include most cursorial animals, including savannah fauna). Thus, human hunters must be able to chase animals for long periods of time, and must therefore have an efficient mechanism for radiating body heat. Upright posture exposes less surface area of the body to direct solar radiation, and subcutaneous sweat glands developed, providing such a cooling mechanism. A more recent theory for human hair loss has to do with a possible period of bipedal wading in a salt marsh in the Danakil region of Ethiopia, which occurred in the hominid lineage, between 5 and 7 million years ago. As a wading animal, it was more efficient to develop short body hair and a layer of subcutaneous fat for streamlining and insulation in the aquatic environment; the eccrine sweat glands developed later after the hominids left the water. This is why most hairless mammals are aquatic (dolphins, dugongs, whales), had an aquatic period in their pasts (elephants, rhinoceroses, pigs) or have very short fine fur because of brief periods back out of the water (seals, sea lions). There is a hypothesis that claims humans are no exception to this rule of hairlessness through means of a marine transition; see Aquatic Ape Theory.
Typically, humans have more hair on the top of the head, because this region of the body was exposed to solar radiation at all times, even when wading, and also hair where extremities meet the torso (axillary (arm-pit) hair, and pubic hairPubic hair is hair in the frontal genital area and in the crotch, and sometimes at the top of the inside of the legs; these areas form the pubic region. A slang term is bush . Although fine vellus hair is present in the area in childhood, the term pubic h), on the eyeThis article refers to the sight organ. See Eye (disambiguation) for other usages. human eye. Note that not all eyes have the same anatomy as a human eye. An eye is an organ that detects light. Different kinds of light-sensitive organ are found in a varielids and above them (eyebrows). In most societies people shave, style or adorn their hair for aesthetic reasons.
Sometimes, the term body hair is used, to distinguish hair on the body from hair on the head. All hairs alternate regular periods of growth and dormancy. During the growth portion of the cycle, hair follicles are long and bulbous, and the hair advances outward at about a third of a millimeter per day. After three to six months, body hair growth stops (the pubic and axillary areas having the longest growth period). The follicle shrinks and the root of the hair rigidifies. Following a period of dormancy, another growth cycle starts, and eventually a new hair pushes the old one out of the follicle from beneath. Head hair, by comparison, grows for a long duration and to a great length before being shed . Anthropologists speculate that the functional significance of long head hair may be adornment, a by-product of secondary natural selection once other somatic hair had been lost.
Unlike other animals, human beings often have their hair cut or remove it by shaving or other means.
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